Roller element bearings

Paul Boughton

Schaeffler (UK) has unveiled the 'ball roller', a rolling element bearing that not only saves design space, but also reduces friction and enables bearing width and mass to be decreased by some 20 per cent

The prototype 'ball roller' bearing incorporates bearing elements that are spherical, but which have their sides cut off. The result is a range of ball roller elements that offer all the axial load handling capabilities of fully spherical balls, but more importantly, allow overall bearing width and mass to be decreased by around 20 per cent and friction to be reduced.

The 'ball roller', which was developed over a period of almost two years, represents a genuine breakthrough in new bearing technology and should generate huge interest from the automotive industry and other industrial sectors, particularly where compactness of bearings is a key design consideration. Manufacturers of wheel bearings and automotive transmission systems such as gearboxes, are potential beneficiaries of the new technology.

According to Heinrich Hofmann, development engineer special projects at Schaeffler Group: “The idea for the ball roller came from our development engineers, who were testing ball bearings and discovered that the spherical balls tended to roll about a single axis and made no use of those areas adjacent to this axis.”

In a typical ball bearing, only 70 per cent of the ball width is utilised, so the outer 15 per cent to the left and right of the ball diameter can be considered redundant.

This discovery led the company to the idea of cutting off this 'redundant' material from the sides of the balls. After six months of advanced 3D modelling, finite element analysis (FEA) simulation, and dynamic modelling using the firm's own proprietary bearing dynamic modelling software, the first prototypes were developed. Initially, these were based on single row bearings, then double row roller bearings, then four-row bearings, to be used as vehicle wheel bearings.

The development team realised that cage design would be critical to the new bearing concept. As Hofmann explains: “Because the balls cannot be allowed to greatly change their rotation axes, cage design was crucial. The critical conditions occur during initial rotation. Once the bearings are moving under conditions of speed and load, they become self-locating, like a bicycle wheel.”

The company's KXR range of bearings were tested using the new concept and a novel cage design. The pocket bases of the cage were designed so that the ball roller, under load, aligned itself freely as a function of the contact angle.

Aside from the obvious benefits from being able to manufacture thinner bearings - or with the same thickness but able to carry higher loads - it is possible to get more bearing elements into the same size of bearing. The balls are loaded by positioning the inner race eccentrically with respect to the outer race. Because the ball rollers have their sides cut off, it is possible, for example, to get 11 ball roller elements into the company's '6207' basic bearing rather than nine fully round balls.

In addition, having moved away from a fully spherical shape, it is possible to give the roller a logarithmic profile, since the rotational axis is always perpendicular to the variable contact angle. The oscillation conditions - the 'kiss' between the roller and the bearing groove - therefore do not change. If the load ratio changes from axial to radial and the contact angle changes as a result, the oscillation 'creeps' in an optimum manner with the change in load.

All computer-based simulations and running tests at Schaeffler have demonstrated that the rollers rotate about their intended rotational axes. Due to their moments of inertia, the rollers are more quickly stabilised than fully spherical balls by 'gyroscopic effects' similar to those that make bicycle pedals more stable at higher speeds.

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